A manipulator utilized in an industrial robot or the like determines the spatial position of an end effecter that moves, inspects, and processes an object, by controlling an arm section connected to the end effecter. It is necessary to have a high degree of freedom in order to allow the end effecter to move to an arbitrary spatial position. Among such manipulators, a so-called Stewart platform type manipulator (a degree of freedom: 6) is known in which 6 direct-acting expandable parallel link manipulators each comprising an actuator are connected to the end effecter. This manipulator has been put to practical use for machine tools.
However, if the manipulator is used for a certain operation (for example, packing, aligning, or sorting articles, loading and unloading work pieces onto and from a machine tool, or inspecting articles), the end effecter need not be rotated in a space. In this case, it is sufficient for the end effecter to be able to move in a particular planar direction and a particular height direction. That is, the manipulator has only to have a degree of freedom of 3. The Examined Japanese Patent Application Publication (Tokko-Hei) No. 4-45310 describes an example of a manipulator having a degree of freedom of 3 and which can be used for applications such as those described above.
FIG. 10 shows the principle of configuration of this conventional manipulator. This manipulator 10 is designed so that three arms 4 are attached to a base section 1 acting as a reference so that the leading ends of the three arms 4 support an end effecter 9. The arms 4 are attached so as to extend radially from the base section 1 in three directions at substantially equal angular intervals. Each arm 4 is composed of a single link section 5 having one end connected to a rotating shaft 2 provided on the base section 1 and a parallel link section 7 connected to the other end of the single link section 5 via another rotating shaft 6 and composed of two bars of the same length. Three rotating shafts 2 of the base section 1 are arranged in the same plane and are installed so that the rotating shafts 6 are parallel with the corresponding rotating shafts 2. The parallel link section 7 is connected to each of the rotating shaft 6, located at the upper end, and the end effecter 9, located at the lower end, using universal joints 8a, 8b such as ball joints or Cardan joints which have a degree of freedom of 2 or more. The universal joint 8b, which connects the leading end of the parallel link section 7 of each of the three arms 4 to the end effecter 9, is arranged in the same plane (hereinafter referred to as the “particular plane”). Driving means 3 such as a servo motor is connected to each of the rotating shaft 2, provided on the base section 1, in order to rotationally drive the rotating shaft 2 around its axis.
In the manipulator 10, configured as previously described, the single link section 5 can be rotationally moved integrally with the rotating shaft 2 of the base section 1 around the axis of rotating shaft 2. The single link section 5 and the parallel link section 7 rotationally move around the axis of the rotating shaft 6 so as to change the angle between the single link section 5 and the parallel link section 7. Furthermore, the parallel link section 7 can be rotationally moved around the universal joint 8 relative to the rotating shaft 6, located at the upper end of the manipulator, and the end effecter 9, located at the lower end of the manipulator. Accordingly, by using the driving means 3 to rotationally drive the three rotating shafts 2 of the base section 1, the postures of the three arms 4 can be controlled to move the end effecter 9 to a desired position. In this regard, owing to the organizational characteristics of the parallel link section 7, whatever posture the parallel link section 7 assumes, the universal joint 8b, located at the lower end, maintains a posture parallel with the rotating shaft 6, located at the upper end. Thus, the end effecter 9, supported at the leading ends of the three arms 4 always maintains a posture parallel with the particular plane when its position is changed by changing the postures of the arms 4. That is, the manipulator 10 can be moved to an arbitrary position while always keeping the end effecter 4 parallel with the particular plane.
The conventional manipulator 10 uses the mechanism in which the load of the entire arm 4 is supported by the rotating shaft 2, located at the proximal end of the arm 4, and in which the rotating shaft 2 is rotationally driven to control the posture of the arm 4. Thus, disadvantageously, the single link section 5 and parallel link section 7, constituting the arm 4, undergo heavy loads. For example, it is assumed that an article is transferred by installing an appropriate task device on the end effecter 9. Then, when the driving means 3 rotationally drives the rotating shaft 2 in order to transfer an article, a large bending moment acts on the link members of the single link section 5 and parallel link section 7 because the postures of the arms 4 are changed while the load of the article gripped by the end effecter 9 is acting on the arms 4. As a result, deformation such as bending occurs in the link members, thus hindering the article from being transferred to an exact position. Further, the life expectancy of the link members is shortened. These disadvantages are more marked as the load of the article to be handled increases. Consequently, it has hitherto been difficult to efficiently and accurately transfer heavy articles.
The parallel link manipulator is a mechanism in which an end effecter such as a motion base is connected to a fixed base using a plurality of parallel links. Some of the joints used in the parallel link manipulator are driven joints, while the others are driving joints. It is expected that the parallel link manipulator enables rigid and accurate linear high-speed movement from the current position to a target position, which movement is difficult to achieve with conventional serial mechanisms. However, since the parallel mechanism has the driven joints, forward kinematic calculations are difficult. Accordingly, in most cases, control using task coordinate variables is not carried out. This is particularly because an inverse matrix J−1 of a Jacobian matrix J is required to convert driving joint coordinate variables into estimated values for task coordinate variables. As is well known, the elements of the Jacobian matrix are not constants but functions of the task coordinate variable. It is thus a heavy mathematical burden to evaluate the values of the inverse matrix in connection with the respective values of the task coordinate variables. Such a mathematical burden may hinder the parallel mechanism from being put to practical use.
It is a basic object to eliminate the need for the inverse matrix J−1 of the Jacobian matrix in determining task coordinate variables. An additional object of aspect of the invention set forth in claim 6 is to eliminate the need for an inverse matrix J−T of a transposed matrix of the Jacobian matrix in providing feedback to driving joints. An additional object of aspect of the invention set forth in Claim 7 is to provide a parallel mechanism that can rigidly move an end effecter while maintaining a constant posture of the end effecter.